Support for working with `set -u` was broken by 94160d62.  Egg on my
face; I'm the one who wants `set -u` support, and I'm the author of
that commit!

libmakepkg does not work with `set -u`; but mostly because of the include
guards!  So we just need to temporarily disable `set -u` (nounset) while
loading libmakepkg.  Instead of introducing a new variable, just store the
initial nounset status in _INCLUDE_COMMON_SH; rather than a useless
fixed-string "true".

While we're at it, disable POSIX-mode (just in case we're running as "sh"
instead of "bash"), since libmakepkg uses bash-isms that won't parse in
POSIX mode.